Spinel cobalt-based binary metal oxides as emerging materials for energy harvesting devices: synthesis, characterization and synchrotron radiation-enabled investigation

The synthesis and characterization of spinel cobalt-based metal oxides (MCo2O4) with varying 3d-transition metal ions (Ni, Fe, Cu, and Zn) were explored using a hydrothermal process (140 °C for two hours) to be used as alternative counter electrodes for Pt-free dye-sensitized solar cells (DSSCs). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed distinct morphologies for each metal oxide, such as NiCo2O4 nanosheets, Cu Co2O4 nanoleaves, Fe Co2O4 diamond-like, and Zn Co2O4 hexagonal-like structures. The X-ray diffraction analysis confirmed the cubic spinel structure for the prepared MCo2O4 films. The functional groups of MCo2O4 materials were recognized in metal oxides throughout Fourier transform infrared (FTIR) analysis. The local structure analysis using X-ray absorption fine structure (XAFS) at Fe and Co K-edge identified octahedral (Oh) Co3+ and tetrahedral (Td) Co2+ coordination, with Zn2+ and Cu2+ favoring Td sites, while Ni3+ and Fe3+ preferred Oh active sites. Further investigations utilizing the Fourier transformation (FT) analysis showed comparable coordination numbers and interatomic distances ranked as Co–Cu > Co–Fe > Zn–Co > Co–Ni. Furthermore, the utilization of MCo2O4 thin films as counter electrodes in DSSC fabrication showed promising results. Notably, solar cells based on CuCo2O4 and ZnCo2O4 counter electrodes showed 1.9% and 1.13% power conversion efficiency, respectively. These findings indicate the potential of employing these binary metal oxides for efficient and cost-effective photovoltaic device production.


Introduction
Because of the permanent growth in the global population and industrialization, the demand for energy has increased steadily.Limits in fuel-based energy sources make the need for cleaner, sustainable alternative energy sources inevitable.Among all renewable energy sources, solar energy is one of the most impressive ones.Photovoltaic technologies, particularly dyesensitized solar cells (DSSCs), have emerged as key factors in this transition owing to their low cost and easy production processes.Another important parameter that increases the impression of DSSCs is the capability of harvesting energy from even lower-intensive light.
Signicant effort has been devoted to increasing the power conversion efficiency and stability of DSSCs while making them cost-effective.To achieve optimal performance in the DSSCs, the counter electrode commonly incorporates platinum (Pt) on a surface of transparent conductive oxide. 1,24][5] The proposed approaches involve the binding of a sensitizer onto wide bandgap semiconductor substrates such as titanium dioxide, which aims to enhance light absorption capabilities. 6dditional efforts have been made to replace the Pt counter electrode in DSSCs.][9][10][11] An alternative candidate with signicant potential for DSSCs is spinel cobalt-based metal oxides (MB 2 O 4 , M: tetrahedral, B: octahedral sites).Such unique characteristics involve excellent electrical conductivity, efficient charge transfer, and signicantly large surface area.][14][15] The spinel cobalt oxide (Co 3 O 4 ) is one of the most investigated electro-catalyst materials that is widely applied in the counter electrode for DSSCs.The tetrahedral site (Td) is occupied by Co 2+ ions, while the octahedral site (Oh) is occupied by two Co 3+ ions. 168][19] Recent studies report that replacing the 3rd transition metal ions, such as Ni and Zn ions, with the Co 2+ in spinel cobalt oxide (Co 3 O 4 ) enhances their electrical conductivity. 20For instance, ZnCo 2 O 4 and NiCo 2 O 4 exhibited superior electrical conductivity compared to Co 3 O 4 .Zn 2+ ions are benecial for indicating defects in the Td sites., [17][18][19][20] as observed in spinel oxides, which in turn enhances the catalytic activity for the oxygen evolution reaction.Moreover, the large surface and electrical conductivity of NiCo 2 O 4 result in better catalytic activity than spinel metal oxide (Co 3 O 4 ).Also, the Fe 3+ in FeCo 2 O 4 occupies the Oh sites, leading to a shi in energy level near the Fermi level, which improves the activity of spinel metal oxide. 21,22everal reports have integrated the spinel-based cobalt structure into DSSC devices.Interestingly, the Co 3 O 4 counter electrode of PCE ∼8.6% in DSSCs exhibits an excellent catalytic performance towards the iodide electrolyte compared to the Pt standard counter electrode. 23It has been reported also that, Co employed in DSSCs results in enhanced diffusion length and transport within a device. 25The DSSCs based spinel NiCo 2 O 4 nanostructures reported high performance and are comparable to the Pt counter electrode. 26To address, the role of the local structure of the spinel-based cobalt materials in DSSCs performance still needs in-depth investigation.
In the present work, three-dimensionally arrays of spinel cobalt-based metal oxide thin lms of MCo 2 O 4 (M: Ni, Fe, Cu, Zn) were fabricated through hydrothermal deposition to use them as counter electrodes to obtain cost-effective DSSCs.Surface morphology and roughness of the prepared samples were investigated throughout SEM and AFM analysis.The XAFS technique is employed to examine the atomic coordination and interatomic distances by observing the local structure at Fe and Co K-edge.Also, the XANES spectra indicate the prepared samples have a spinel structure.Then, spinel cobalt-based metal oxide thin lms were used in the fabrication of DSSCs as promising counter electrodes, and the photovoltaic performance of DSSCs with MCo 2 O 4 counter electrodes were compared.
The samples were grown hydrothermally using a solution processing route.The synthesis of spinel cobalt-based metal oxide thin lms, MCo 2 O 4 , coated over a cleaned ITO substrate is shown in Fig. 1.The ITO substrate was subjected to cleaning in the acetone for 15 min followed by 15 min in the ethanol under sonication.The preparation procedure was performed by Scanning electron microscopy (SEM, Quanta FEG 450) was conducted to investigate the morphology and size distribution of the metal oxide samples.The surface roughness of the MCo 2 O 4 thin lms was carried out by using an atomic force microscope (AFM, SmartSPM™ 1000), FTIR (Bruker Vertex 80 and Hyperion 2000 microscope) used to investigate the vibrational modes and chemical bonds of metal oxides.The XAFS data were obtained via the BM-08 XAFS/XRF beamline at the Synchrotron-Light for Experimental Science and Applications in the Middle East (SESAME).The beamline was run in decay condition at an energy level of 2.5 GeV, with 300 mA as a maximum current.The X-ray absorption ne structure (XAFS) data were measured on the BM08-XAFS/XRF beamline of the SESAME operated at 2.5 GeV in "decay" mode with a maximum electron current of 300 mA. 27The XAFS spectra of the materials were acquired in transmission mode in the spectral range of Co K-edge (7709 eV) and Fe K-edge (7112 eV) at room temperature.The X-ray beam intensity was measured by ionization chambers lled with an optimal mixture of noble gases at a total pressure of 1.0 bar, and the XAFS data of the samples were acquired from the signals measured at ion chambers subsequently amplied by Stanford picoammeters and digitalized by a voltage to frequency converter, using a double-crystal Si (111) monochromator.The energy was calibrated at the Co K-edge (7709 eV) and Fe K-edge (7112 eV) of the Co and Fe standard metal foils, respectively.The sample was prepared in pallet form (13 mm diameter) by pressing a homogeneous mixture of calculated quantity of nely ground material and polyvinylpyrrolidone (PVP) powder.The amount of material in the pellet was calculated using XAFS mass soware to give an absorption mt ∼1.5, just above the Co K-edge and Fe-K edge absorptions.
The DSSCs were prepared using TiO 2 nanoparticle decorated ITO coated photoanodes.The electrodes were sintered at 450 °C in air ambient for 30 min to remove contaminations from the TiO 2 nanoparticle surfaces.And then was immersed in N719 dye for a night.The photoanodes were washed with ethanol and dried using a heat gun.MCo 2 O 4 coated ITO counter electrodes and dye-coated TiO 2 photoanodes were sealed together and an electrolyte (iodide/tri-iodide in a nitrile solvent) was used to ll the space between two electrodes.The performances of DSSCs were measured under a class A solar simulator (ABET Technology Sun2000) with an AM 1.5 G lter and 100 mW cm −2 light intensity using a Keithley 2425 source meter.

Results and discussion
The DSSCs device performance is directly inuenced by the morphology of the counter electrode in particular redox/ oxidation electrolyte at the interface.SEM images can give insight into crystal arrays and defective surfaces of spinel cobalt oxides.The surface morphologies of as-prepared MCo 2 O 4 thin lms are illustrated in Fig. 2. Interestingly, the morphology of the prepared samples can be correlated to the variations in source metal ions.Fig. 2 demonstrates high-pressure conditions controlled by the (111) plane, devoid of any preferential axis for selective growth. 28ig. 2(e) and (f) displays a hierarchical structure of NiCo 2 O 4 nano-sheets observed with hollow arrays.Fig. 2(g) and (h) depicts a twin hexagonal conguration of nano-block ZnCo 2 O 4 arrays observed with non-uniform distribution due to anisotropic growth along [100] and [001] directions. 29It is impressive to nd that different Zn 2+ , Fe 2+ , Ni 2+ , and Cu 2+ reacted species tend to form spinel structures in a cubic crystal phase.The intricate reaction pathways of these species during the hydrothermal process drive the formation of distinct morphologies of spinel metal oxides.Initially, the decomposed reacted species undergo a recrystallization process to form MCo 2 O 4 with preferential growth on selective planes, depending on the nature of the ions.The synthesis of MCo 2 O 4 is driven by the formation of M(OH) 2 and Co(OH) 2 before transforming into MCo 2 (OH) 6 complex compound, as demonstrated by the NR Chodankar group. 30Alternatively, another reaction route suggests that the formation pathway is governed by the formation of Co 3 O 4 and MO, which are then reduced to the MCo 2 O 4 structure. 31The annealing process reduces the OH − ions in the complex compound to the oxide form, resulting in the obtaining of bimetal oxides.
The HMT acts as an alkaline source that assists in the formation of M-Co precursor.During the hydrolysis of HMT, NH 3 is released, making the solution alkaline.This process also produces OH − ions in the hydrothermal synthesis, which react with Zn 2+ , Fe 2+ , Ni 2+ , Cu 2+ , and Co 2+ ions to form spinel metal oxide.Furthermore, studies have found HMT useful for obtaining a crystalline spinel metal oxide structure.The alkaline environment facilitated by HMT is preferable for precipitating M 2+ and Co 2+ ions during hydrothermal deposition, accelerating the formation of the crystalline MCo 2 O 4 structure. 32,33he surface area and roughness of the electrodes can provide signicant insight into understanding the charge transfer mechanism and overall device performance.The AFM imaging was performed for the MCo 2 O 4 thin lms and illustrated in Fig. 3.The AFM images of the prepared CuCo 2 O 4 thin lm exhibit a high root mean square (RMS) roughness of 0.16 mm and a large surface area of 38.74 mm 2 .While the FeCo 2 O 4 thin lms show a polygon-shaped structure with an overall RMS of 0.03 mm and a surface area of 29.9 mm 2 .The phase image demonstrates a variation between -50°and 20°indicating low phase separation at the grain boundaries.The NiCo 2 O 4 nanosheets thin lms depict a large surface area of 58.87 mm 2 and RMS of 0.26 mm with an average thickness of nearly 20 nm, while a high roughness was observed for the hexagonal structure of ZnCo 2 O 4 with an RMS of 0.35 mm and surface area of 34.9 mm 2 .The hexagonal plates of ZnCo 2 O 4 are stacked in a nonuniform structural array.
In DSSCs, the device performance is highly dependent on the crystal quality of the counter electrode based a binary metal oxide lms.The cubic spinel structure was conrmed for various metal oxide lms from crystallographic X-ray diffraction plots in Fig. S1.† As seen, the diffracted XRD peaks of FeCo 2 O 4 lm match the (220), (311), ( 400 The UV-vis-NIR spectral of the spinel metal oxide lms plots in Fig. S2 † and the corresponding extrapolated absorption band gap energies (E g ) were determined according to Tauc plot.The NiCo 2 O 4 nanosheets exhibit higher absorbance compared to the other spinel materials oxides followed by crystal CuCo 2 O 4 nanoleaves.The straight-line interception yields two band gap energies of 1.98 and 2.50 eV for ZnCo 2 O 4 , while the obtained band gap energies of 2.00 and 3.35 eV for NiCo 2 O 4 in agreement with the reported values of 2.00 and 3.30 eV. 34The valence band in NiCo 2 O 4 was constructed from O 2p orbital and the conduction band from 3d orbitals of Ni and Co.And has an electron conguration of tetrahedral high spin Co 2+ (eg 4 t 2 g 3 ), octahedral low spin Co 3+ (t 2 g 6 ), and Ni 3+ (t 2 g 6 eg 1 ).Therefore, the electron can be excited from Co-3d-t 2g to the partially lled Co 3d-eg orbital.Consequently, the presence of two band gap energies can be assigned to high spin and low spin of Co 3+ in the spinel structure. 34The spinel CuCo 2 O 4 band gap energies was 1.88 eV and 3.0 eV and FeCo 2 O 4 yields 2.18 eV and 2.82 eV.The determined values are close to those reported in the literature. 35,36he solution processed DSSCs device via hydrothermal usually contains -OH functional group.To verify the presence of the -OH group and other functional groups that could inuence the device performance, the FTIR prole illustrated in Fig. 4  and 700 cm −1 may be associated with octahedral and tetrahedral sites. 37Furthermore, stretching modes located at approximately 882 cm −1 and 1000 cm −1 are assigned to the C-O group.Furthermore, the weak vibrational mode at 1277.8 cm −1 and 1299.1 cm −1 is linked to the NO 3 − group. 38ditional stretching characteristics identied at 1598.8 cm −1 and 1600 cm −1 were assigned to COO and C]O molecules. 39he stretching modes at 2931.7 cm −1 , 2936.8 cm −1 , 3785.8 cm −1 , and 3795.8 cm −1 are attributed to the O-H functional group. 40,41A higher content of OH-molecules was noticed in the case of ZnCo 2 O 4 associated with a prominent shi in the OH vibration mode.The local structure of the MCo 2 O 4 thin lms could play an effective role in the DSSC device performance.The applied XAFS integrates to resolve the local structure of spinel cobalt oxide devices.The XANES spectra at Co absorption K-edge are presented in Fig. 5 of the prepared samples.Resolving the FeCo 2 O 4 structure at the Co absorption K-edge is elusive and therefore XANES is applied at the Fe absorption K-edge.According to the XANES data, the Co absorption K-edge was (7722.53eV) for NiCo 2 O 4 , (7724.02eV) for CuCo 2 O 4 and (7722.18eV) for ZnCo 2 O 4 .The observed Co absorption K-edge aligns with the value reported in the literature at 7721.4 eV, while the Fe absorption K-edge was 7125.53 eV for FeCo 2 O 4 .The oxidation state of the cobalt cation is Co 3+ and the observed XANES spectrum reveals the spinel structure of the prepared samples. 42,43he observed XANES spectrum reveals a low-intensity preedge peak at A, which can be assigned to 1s / 3d quadrupole transition in spinel cobalt oxide (Co 3 O 4 ). 44A transition edge (shoulder region) was observed at B, along with the prominent white-line peak of high intensity at C. These transition edges at B and C can be attributed to resonance peaks. 45,46The white-line region can be ascribed to the 1s / np transition. 46ollowing that, minor peaks of low intensity were observed at D and E. The broadening of peak D, in particular, in FeCo 2 O 4 can be attributed to the coordination atoms in the vicinity of the Co absorber atom.Moreover, two peaks at E and F indicate the presence of multiple scattering associated with the coordination of the medium-range structure surrounding the Co absorber atom. 47,48Furthermore, two peaks at G and H were observed in the XANES spectrum coinciding with the ZnCo 2 O 4 sample, which can be attributed to the existence of multiscattering effects.
To facilitate a comprehensive quantitative analysis of the local atomic structure of the MCo 2 O 4 nanomaterials, the EXAFS tting in R-space was conducted. 49The experimental EXAFS data in R-space in the range of 1.0 to 5.0 (Å) with Hanning window and k range of 3.0 Å −1 were analyzed to the best t.The initial four high-ranking single scattering paths of Co-O and M-Co bond pairs were included in the t.The passive electrons reduction factor S 0 2 and energy shi E 0 were set similarly for all the paths in the t, and the mean-square relative displacement s 2 and interatomic distance R were rened relatively to get the best t result.Fig. 6      The above-reported XANES (Fig. 5b) pre-edge of NiCo 2 O 4 suggested the presence of T d Co 2+ symmetry. 55It can be preliminary concluded that, the NiCo 2 O 4 is close to the spinel Co 3 O 4 structure and the oxidation state for both is 8/3 as oxidation state corresponding to the presence of O h Ni 3+ . 56The FT analysis of NiCo 2 O 4 reveals a peak at the lowest interatomic distance of 1.94 Å assigned to Co-O 1 of O h (Co 3+ O 6 ) and Co-O 2 of T d (Co 2+ O 4 ). 57The next FT peak at 2.9 Å corresponds to Co-Co 1 (2.The working mechanism of DSSCs in this study, as illustrated in Fig. 7, involves several key components and processes.
The TiO 2 nanoparticle-decorated ITO-coated photoanode serves as the light-harvesting layer.When light is absorbed by the sensitizer dye (here N719), it excites electrons from the dye into the conduction band of the TiO 2 .These photo-generated electrons then move through the TiO 2 nanoparticles and are collected at the ITO substrate, owing into the external circuit to generate electric current.The oxidized dye molecules are regenerated by electrons from the iodide (I − ) in the electrolyte, which is subsequently oxidized to tri-iodide (I 3     mechanisms associated with these materials.Factors such as differences in catalytic activity, electronic structure, and surface morphology may contribute to the reduced efficiency of photogenerated charge carriers and subsequent lower J SC values.9][60] For instance, Kaya et al. hydrothermally sensitized CuCrO 2 delofossite oxide nanoparticles to use them as photocathode in the fabrication of high efficient tandem p-n photoelectrochemical cells using CuCrO 2 delafossite semiconductors as photocathodes coupling with traditional n-type TiO 2 based photoanodes. 59They reported the efficiency of solar cells between 1.67 and 2.33% for various annealing temperatures. Considering the structural properties examined in this study across various spinel metal oxides, the samples consistently demonstrated high crystallinity, primarily in the cubic phase.Notably, the identication of active structural planes in these samples indicates a correlation between electrochemical activity and charge injection with these specic planes.For instance, the (311) plane was prominent in the ZnCo 3 O 4 has a good electro-catalytic activity towards reduction of I 3 − ions into I − ions. 24Furthermore, zinc substitution for Co in cobalt oxide(II,III) results in the spinel structure of Zn-Co-O with p-type conductivity.The hole transport layer of Zn-Co-O

Fig. 1 A
Fig.1A schematic illustration depicting the deposition procedures of spinel cobalt-based samples.
Fig. 2 SEM images of the obtained (a and b) CuCo 2 O 4 thin film (c and d) FeCo 2 O 4 thin film (e and f) NiCo 2 O 4 thin film and (g and h) ZnCo 2 O 4 thin film.

Fig. 4
Fig. 4 FTIR profile of the prepared binary metal oxide films MCo 2 O 4 .

Fig. 5
Fig. 5 XANES spectra collected at Fe and Co K-edge of (a) NiCo 2 O 4 , (b) ZnCo 2 O 4 , and (c) CuCo 2 O 4 collected at Co K-edge and (d) FeCo 2 O 4 collected at Fe kedge; inset figures illustrate the corresponding first derivative of the absorption spectra.
T d Co 2+ sites.According to the reported data, the Fe, Cu, and Ni cations exhibit a tendency to occupy the octahedral sites (O h ) and leave tetrahedral (T d ) sites in spinel Co 3 O 4 . 50On the other side, the Co 2+ ions in the Co 3 O 4 compound are substituted by Zn 2+ ions, which exhibit tetrahedral symmetry (T d ), leading to the formation of ZnCo 2 O 4 . 46Interestingly, the ZnCo 2 O 4 tends to form a spinel structure as the Zn 2+ occupy the tetrahedral sites (T d ) and leaving the Co 3+ at octahedral sites (O h ).51The crystal ZnCo 2 O 4 has a spinel structure (A 2+ Co 2 3+ O 4 , where, A is tetrahedral and B is octahedral sites) closely similar to the Co 3 O 4 structure. 51Indeed, the FT shows Co-O 1 (1.966Å) interaction, which is characteristic of O h Co 3+ .Also, the Co-Zn 1 bond suggested T d Zn 2+ and O h Co 3+ in the spinel structure of ZnCo 2 O 4 .The Co-O 1 and Co-Co 1 own octahedral symmetry.This nding shows a good agreement with the previous reports that conrmed the CoO 6 structure has octahedral symmetry, whereas the CoO 4 exhibits tetrahedral symmetry. 52Furthermore, the results imply that the Zn 2+ cations have a tendency to occupy active sites characterized by a low oxidation state, whereas Co 3+ cations are le in octahedral locations.It is clear that T d Zn 2+ and O h Co 3+ ions were presented in the crystalline structure of the ZnCo 2 O 4 spinel structure.Moreover, the FT rst peak at 1.900 Å interatomic distance in spinel CuCo 2O 4 structure corresponds to the Co-O 1 (1.917Å) of Oh Co 3+ and Co-O 2 (2.058 Å) of T d Co 2+ .The results in accordance with the previous reports. 51,53The FT peak at 3.200 Å is correlated to Co-Co 1 (3.237Å) and Co-Cu1 (3.462 Å) bond interactions.It has been reported that the Co-Co interactions can be found in Oh and Td as Co 3+ -Co 3+ (2.85 Å), Co 3+ -Co 2+ (3.346 Å), and Co 2+ -Co 2+ (3.495 Å). 54 Therefore, the Co-Co1

Fig. 6
Fig. 6 k 3 -weighted EXAFS data with the best fit from Co and Fe K-edge absorption, FT, and the Re (FT) of the EXAFS data, respectively for (a-c) FeCo 2 O 4 , (d-f) ZnCo 2 O 4 , (g-i) Cu Co 2 O 4 and (j-l) NiCo 2 O 4 samples.
988 Å) and 4Co-Ni 1 (2.835Å) interactions.It's evident the Co-Co 1 is characteristic of Oh Co 3+ and the Co-Ni1 coincides characteristic of O h Co 3+ and the Co-Ni 1 coincides with O h Co 3+ and O h Ni 3+ .In general, the Co-O interatomic distances at T d or O h sites are shorter than M-O distances which is correlated with a small atomic radius of Co 3+ in comparison with M 2+ .It is obvious that the Co-O 1 and Co-O 2 in ZnCo 2 O 4 are longer compared to other spinel structures.It is worth noting that, the interatomic distances of M-Co introduced in Table 1 are observed as Co-Cu > Co-Fe > Zn-Co > Co-Ni.

−).
The tri-iodide ions diffuse towards the spinel cobalt-based metal oxide MCo 2 O 4 thin lm counter electrodes, where they are reduced back to iodide ions, completing the circuit.This process is facilitated by the high electrocatalytic activity of the MCo 2 O 4 counter electrodes, which efficiently catalyzes the reduction of tri-iodide.The high surface area and conductivity of the MCo 2 O 4 thin lms can play an important role in efficient electron transfer and low charge transfer resistance, contributing to the high performance of the DSSCs.Additionally, the chemical and thermal stability of MCo 2 O 4 provides durability and longterm performance stability, making it an excellent choice for counter electrodes in high-performance DSSCs.

Fig. 8
Fig.8shows current density-voltage (J-V) plots of the DSSCs fabricated using sensitized MCo 2 O 4 based counter electrodes in comparison with Pt-based one as reference solar cell under illumination of 100 mW cm −2 .Table2summarizes the obtained results and highlights signicant differences among the examined thin lms.As presented in Table 2, the DSSC with a counter electrode based on CuCo 2 O 4 nano-leave gives the best photovoltaic response with a J max of 4.23 mA cm −2 and a V max of 550 mV which results in 1.9% power conversion efficiency.It is also seen that the open circuit voltage (V OC ) value of this device (705.8 mV) superior to the one for conventional DSSC with Pt electrode (664.1 mV).The second-best solar cell efficiency is reported as 1.13% for the DSSC obtained using ZnCo 2 O 4 hexagonal-like structures.The V OC value of this device with 642.0 mV is very close to the cell with Pt electrode.The other two solar cells obtained from NiCo 2 O 4 nanosheets and FeCo 2 O 4 diamond-like structures have lower PCE values of 0.27 and 0.15%, respectively.The lower short-circuit current density (J SC ) values observed in our study for DSSCs utilizing alternative counter electrodes, such as CuCo 2 O 4 , FeCo 2 O 4 , NiCo 2 O 4 , and ZnCo 2 O 4 , compared to conventional Pt-based DSSCs.It suggests potential limitations in transport mechanisms associated with these materials.Factors such as differences in catalytic activity, electronic structure, and surface morphology may contribute to the reduced efficiency of photogenerated charge carriers and subsequent lower J SC values.Similar studies have been performed for various metal oxides including binary metal oxides.[58][59][60]For instance, Kaya et al. hydrothermally sensitized CuCrO 2 delofossite oxide nanoparticles to use them as photocathode in the fabrication of high efficient tandem p-n photoelectrochemical cells using CuCrO 2 delafossite semiconductors as photocathodes coupling with traditional n-type TiO 2 based photoanodes.59They reported the efficiency of solar cells between 1.67 and 2.33% for various annealing temperatures.Considering the structural properties examined in this study across various spinel metal oxides, the samples consistently demonstrated high crystallinity, primarily in the cubic phase.Notably, the identication of active structural planes in these samples indicates a correlation between electrochemical activity and charge injection with these specic planes.For instance, the (311) plane was prominent in the ZnCo 2 O 4 structure, the (220) plane in CuCo 2 O 4 , both (311) and (220) planes in NiCo 2 O 4 , and the (220) plane in FeCo 2 O 4 .Furthermore, distinctive characteristics and morphologies were observed in the hexagonal structure of ZnCo 2 O 4 and the nanosheet morphology of CuCo 2 O 4 , which are thought to enhance the photocurrent and efficiency of DSSCs.
Fig.8shows current density-voltage (J-V) plots of the DSSCs fabricated using sensitized MCo 2 O 4 based counter electrodes in comparison with Pt-based one as reference solar cell under illumination of 100 mW cm −2 .Table2summarizes the obtained results and highlights signicant differences among the examined thin lms.As presented in Table 2, the DSSC with a counter electrode based on CuCo 2 O 4 nano-leave gives the best photovoltaic response with a J max of 4.23 mA cm −2 and a V max of 550 mV which results in 1.9% power conversion efficiency.It is also seen that the open circuit voltage (V OC ) value of this device (705.8 mV) superior to the one for conventional DSSC with Pt electrode (664.1 mV).The second-best solar cell efficiency is reported as 1.13% for the DSSC obtained using ZnCo 2 O 4 hexagonal-like structures.The V OC value of this device with 642.0 mV is very close to the cell with Pt electrode.The other two solar cells obtained from NiCo 2 O 4 nanosheets and FeCo 2 O 4 diamond-like structures have lower PCE values of 0.27 and 0.15%, respectively.The lower short-circuit current density (J SC ) values observed in our study for DSSCs utilizing alternative counter electrodes, such as CuCo 2 O 4 , FeCo 2 O 4 , NiCo 2 O 4 , and ZnCo 2 O 4 , compared to conventional Pt-based DSSCs.It suggests potential limitations in transport mechanisms associated with these materials.Factors such as differences in catalytic activity, electronic structure, and surface morphology may contribute to the reduced efficiency of photogenerated charge carriers and subsequent lower J SC values.Similar studies have been performed for various metal oxides including binary metal oxides.[58][59][60]For instance, Kaya et al. hydrothermally sensitized CuCrO 2 delofossite oxide nanoparticles to use them as photocathode in the fabrication of high efficient tandem p-n photoelectrochemical cells using CuCrO 2 delafossite semiconductors as photocathodes coupling with traditional n-type TiO 2 based photoanodes.59They reported the efficiency of solar cells between 1.67 and 2.33% for various annealing temperatures.Considering the structural properties examined in this study across various spinel metal oxides, the samples consistently demonstrated high crystallinity, primarily in the cubic phase.Notably, the identication of active structural planes in these samples indicates a correlation between electrochemical activity and charge injection with these specic planes.For instance, the (311) plane was prominent in the ZnCo 2 O 4 structure, the (220) plane in CuCo 2 O 4 , both (311) and (220) planes in NiCo 2 O 4 , and the (220) plane in FeCo 2 O 4 .Furthermore, distinctive characteristics and morphologies were observed in the hexagonal structure of ZnCo 2 O 4 and the nanosheet morphology of CuCo 2 O 4 , which are thought to enhance the photocurrent and efficiency of DSSCs.Conclusions The spinel cobalt-based metal oxides of MCo 2 O 4 (M: Ni, Cu, Fe, or Zn), were synthesized through the hydrothermal growth technique on ITO substrate to use them as counter electrodes in the fabrication of Pt free DSSCs.The oxides displayed a uniform and homogeneous surface morphology.Interestingly, NiCo 2 O 4 exhibits nanosheets, Fe Co 2 O 4 displays diamond shapes, CuCo 2 O 4 shows nanoleaves, and ZnCo 2 O 4 presents a hexagonal shape.These morphologies exhibit unique structural features characterized by a signicantly large surface area and the existence of surface defect sites to enhance chemical reactivity.To probe the local structure of the prepared samples, EXAFS data was carried out at Fe and Co K-edge.The absorption Co K-edge was close to the 7721.4eV value reported in the literature.The Fe Kedge was 7125.53 eV for FeCo 2 O 4 .The pre-edge peak in XANES of the prepared samples suggested T d Co 2+ .The shape of the XANES spectrum reveals the spinel structure of the obtained samples.The FT reveals a Co-O 1 coincides with O h Co 3+ and Co-O 2 is characteristic for T d Co 2+ .On the contrary, Fe-O 1 reveals T d Fe 2+ and Fe-O 2 associated with O h Fe 3+ .The interatomic distances evaluated from FT analysis of M-Co were observed as Co-Cu > Co-Fe > Zn-Co > Co-Ni.In addition, it is reported that these spinels cobalt-based binary metal oxides, especially CuCo 2 O 4 nanoleaves and ZnCo 2 O 4 hexagonal-like structures are promising candidates for the low cost and high efficient DSSCs with 1.9 and 1.13% power conversion efficiency values, respectively.
plots k 3 -weighted EXAFS with the best t, and the FT, respectively.The t parameters with details are listed in Table 1.The FT analysis of spinel FeCo 2 O 4 structure shows a strong peak around 1.500 Å assigned to the Fe-O 1 (2.239Å) coincides with T d Fe 2+ and Fe-O 2 (1.963 Å) assigned to O h Fe 3+ .The Fe-Co 1 and Fe-Fe 1 bonds are observed near the 3 Å which reveals the Co 2+ cations are substituted by Fe 2+ cations in the spinel Co 3 O 4 structure (FeCo 2 O 4 ).The Fe-Co 1 can be resolved to O h Fe 3+ and

Table 1
EXAFS fitting parameters, including coordination number (N), mean coordination shell radii (R), mean square relative displacements (MSRDs) or Debye-Waller factor (s 2 ), amplitude reduction factor (S o2), photoelectron energy (E o ) and goodness of the fit (R factor ) for the MCo 2 O 4 binary metal oxides Fig. 7 Schematic diagram of MCo 2 O 4 counter-electrode-based DSSC.
Table 2 summarizes the obtained results and highlights signicant differences among the examined thin lms.As presented in Table 2, the DSSC with a counter electrode based on CuCo 2 O 4 nano-leave gives the best photovoltaic response with a J max of 4.23 mA cm −2 and a V max of 550 mV which results in 1.9% power conversion efficiency.It is also seen that the open circuit voltage (V OC ) value of this device (705.8 mV) superior to the one for conventional DSSC with Pt electrode (664.1 mV).The second-best solar cell efficiency is reported as 1.13% for the DSSC obtained using ZnCo 2 O 4 hexagonal-like structures.The V OC value of this device with 642.0 mV is very close to the cell with Pt electrode.The other two solar cells obtained from NiCo 2 O 4 nanosheets and FeCo 2 O 4 diamond-like structures have lower PCE values of 0.27 and 0.15%, respectively.The lower short-circuit current density (J SC ) values observed in our study for DSSCs utilizing alternative counter electrodes, such as CuCo 2 O 4 , FeCo 2 O 4 , NiCo 2 O 4 , and ZnCo 2 O 4 , compared to conventional Pt-based DSSCs.It suggests potential limitations in transport